This invention relates generally to the art of medical prostheses, more particularly to human body implants, and especially to such implants that are subjected to sliding or rolling pressure or to a combination of such pressures during function in the human body.
In recent decades, the emphasis in surgical repair of functionally impaired skeletal joints has shifted from fusion of the involved joint and resultant total immobilization thereof to the implantation of synthetic joint components and even artificial, total joint assemblages. Great medical advances have been made using these implants; and the materials selected for the construction of the component parts have commonly been various metals and alloys. Polymeric materials have also been increasingly employed, especially for elements that are incident to sliding, rolling or grinding motion upon articulation of the repaired joint.
Because of their chemical inertness and low friction properties, polyethylene resins have received considerable attention as candidates for anti-friction, human body implants. However, polyethylenes have limitations in the medical environment. For example, they may release surface particles and are known to be susceptible to "cold flow" and resultant loss of intended geometry when subjected to compressive forces over extended periods of time. The higher molecular weight polyethylenes, i.e., those having molecular weights on the order of 400,000 to 600,000 and having linear characteristic, exhibit increased tendencies to incur "cold flow"; but these latter polymers display concomitantly lesser propensities toward stress cracking, after implantation. Heretofore, attempts have been made to compensate for the various deficiencies of polyethylenes by such expediencies as metallic perimeter containments and implantation in cavities prepared to leave a surrounding rim of either bone or a combination of bone and synthetic bone "cement". Undesirable complexities in fabrication and in surgery have been the consequence.
Furthermore, attempts have been made in the past to reinforce various polymers with carbon fibers. However, these efforts have been principally directed either to thermosetting, rather than thermoplastic, resins or to general mechanical, non-medical applications such as bearings, slideways, electrical housings and the like. Moreover, minimal efforts have been devoted to producing polyethylene-carbon fiber composites for medical implants, or other uses, because "fillers" of whatever nature are generally known to have very pronounced and unpredictable effects on the physical properties of polyethylene.